A gene is a segment of DNA that contains the information needed to produce a protein or an RNA molecule with a specific function. It is the basic unit of inheritance.
We have two copies of each gene, one inherited from our mother and the other from our father, except for genes on the sex chromosomes in males, who have one X chromosome and one Y chromosome, and genes in mitochondrial DNA, which we inherit exclusively from our mother. Each version of a gene is called an allele. The concept of a gene has evolved as genetics has advanced.